The invention is related to the field of voltage production circuits, and, more particularly, to contactless chip cards or smart cards having an electrically erasable and programmable memory.
FIG. 1 is a standard diagram of an integrated circuit for contactless smart cards. The integrated circuit comprises a detection circuit 10, a rectifier circuit 20, a filter 30, a low-voltage regulation circuit 34, modulation and demodulation circuits 36, a logic circuit 38 and a memory 40. The detection circuit 10 comprises a resonant circuit, including an antenna loop represented by the turns of a winding 11, and a capacitor 12. The elements 11 and 12 are parallel-connected. The rectifier circuit 20 comprises a four-diode bridge 21 and a capacitor 22 having one of its terminals connected to ground.
The filter 30 has a resistor 31 and a capacitor 32. One of the terminals of the resistor 31 is connected to the common point 23 of one of the outputs of the bridge of diodes 21 and the capacitor 22. Another terminal of the resistor 31 is connected to one of the terminals of the capacitor 32 and to an output 33 of the filter 30. Another terminal of the capacitor 32 is connected to ground.
The low-voltage regulation circuit 34 comprises an input terminal connected to the output terminal 33 of the filter 30 and an output terminal 35 connected to the modulation and demodulation circuits 36, the logic circuit 38 and the memory 40. The modulation and demodulation circuits 36 comprise an input terminal connected to the common point 23 and an output terminal connected to the logic circuit 38 and the memory 40. The low-voltage regulation circuit 34 as well as the modulation and demodulation circuits 36 are formed of transistors, resistors and capacitors in a conventional manner as would be appreciated by the skilled artisan.
In contactless smart cards, the data as well as the power received by the chip are transmitted by the reader in the form of an amplitude-modulated RF signal. The signal received by the card is detected and then rectified by the detection circuit 10 and the rectifier circuit 12 to give a rectified voltage HV1 to the common point 23. The mean amplitude of this rectified voltage HV1 varies according to the mean power received by the card. This mean power depends mainly on the distance between the card and the reader. When the card is close to the reader, about a few centimeters, the amplitude of the rectified voltage HV1 can thus reach 15-20 V, with an appropriate detection circuit.
The filter 30 lowers the level of the rectified voltage HV1. Then, through the regulation circuit 34, a stable, DC low supply voltage Vcc of about 4 V to 5 V is obtained at the output terminal 35. The assembly formed by the detection circuit 10 and the rectification circuit 20, the filter 30 and the low-voltage regulation circuit 34 is thus equivalent to a source of stable voltage that provides a low supply voltage Vcc. This source of stable voltage supplies power to all the circuits of the smart card and especially the modulation and demodulation circuits 36, the logic circuit 38 and the memory 40. At the same time, the modulation and demodulation circuits 36 decode the voltage HV1 and provide a binary signal DATA, containing the data transmitted to the card, the logic circuit 38 and the memory 40.
The memory 40 is an electrically erasable and programmable memory, for example of the EEPROM type. It comprises a memory array 42 storing the data received by the card and a logic circuit 44 to control read and/or write operations in the memory array 42. To carry out a write operation in the memory array 42, it is necessary to have a voltage of about 16 to 18 V. For this, a voltage step-up circuit 46 provides a stable, DC high programming voltage HT from the low supply voltage Vcc. The voltage step-up circuit 46 is, for example, of the charge pump type, and is made of capacitors, transistors and/or diodes according to known techniques.
The smart card receives a limited quantity of energy. Hence, a write operation is most usually performed alone in order to make the best possible use of the power received by the card. Nevertheless, to provide the high programming voltage HT, the charge pump consumes substantial current and hence a large quantity of energy. This causes an overload on the supply voltage source that gives the low voltage Vcc. This overload could lead to a loss of the data received by the card, the erroneous storage of this data or else a break in communication between the reader and the smart card.
Moreover, a conventional charge pump takes up a large silicon surface area of up to 20% of the silicon surface area used for the memory. Indeed, the elements that form this charge pump, especially the capacitors and transistors, are bulky because their sizes are determined so that they can withstand high levels of voltage and power. Lastly, a charge pump is fragile, in particular because the capacitors forming it stand up rather poorly to excessively high voltages.
Given the characteristic disadvantages of the charge pump, it would therefore be worthwhile to eliminate it from a smart card memory, provided however that it is possible to have a stable, DC high voltage source, since this is indispensable for memory write operations.
An object of the invention is to provide an integrated circuit comprising a detection circuit and a rectifier circuit, series-connected to give a rectified voltage, and a low-voltage regulation circuit that receives the rectified voltage and gives a low supply voltage. According to the invention, the integrated circuit also comprises a voltage production circuit that receives the rectified voltage and produces a high voltage different from the low voltage. Thus, with the invention, the high voltage is no longer produced from the low supply voltage but directly from the rectified voltage, thus enabling the elimination of the charge pump commonly used.
The invention has the advantage of separating the voltage production circuit from the low voltage regulation circuit which provides the low supply voltage and from the circuits that are connected to it. Any malfunction in these circuits, due to the overloading of the low-voltage regulation circuit, is thus avoided. The transfer of data between the reader and the card is also avoided since the transfer is managed solely by the modulation and demodulation circuit and the logic circuit.
Furthermore, the voltage production circuit of the invention is far smaller and less fragile, and consumes far less energy than a charge pump as will be seen more clearly here below. Finally, by eliminating the charge pump commonly used in the memories, the invention provides for considerable savings in silicon surface area. This advantage is particularly valuable for applications such as electronic labels which require smaller circuits. According to one use of the invention, the integrated circuit also comprises a memory comprising a memory array, the memory array receiving the low voltage and the high voltage. The memory array is thus powered directly by the voltage production circuit and no longer by a charge pump internal to the memory.
According to one embodiment of the invention, the voltage production circuit is simply a wire. This embodiment is particularly simple to implement and it may be sufficient if the integrated circuit is used always at the same distance, of some centimeters, from the reader. In this case, the variation in the amplitude of the rectified voltage is fairly small.
According to another embodiment, the voltage production circuit comprises a controlled switch that is closed when it receives an active control signal (WRITE), and open otherwise. The control signal (WRITE) is for example a signal given by the logic circuit of the memory, if this logic circuit exists, or else by any other logic circuit of the integrated circuit. This embodiment has the advantage of applying the high voltage, equal to the rectified voltage, only when a control signal is given, namely only when it is necessary. The total consumption of the integrated circuit is thus limited.
According to another embodiment, the voltage production circuit comprises the controlled switch which receives the rectified voltage, and provides an intermediate voltage that is equal to either the rectified voltage if it is closed or to zero if it is open, and a high voltage circuit that receives the intermediate voltage, provides the high voltage and opens or closes the switch to regulate the high voltage. Thus, according to this embodiment, a regulation circuit is placed in series after the switch. Thus, the voltage production circuit continually provides a constant, regulated high voltage.
Finally, according to another embodiment, the voltage production circuit comprises the controlled switch which receives the rectified voltage and provides the intermediate voltage, a high voltage circuit that receives the intermediate voltage, and provides the high voltage and a regulation signal to regulate the high voltage, and a control circuit that receives the regulation signal and the control signal, and provides a closing signal to close or open the controlled switch. According to this embodiment, the voltage production circuit provides a stable DC voltage, particularly well suited tothe programming of a memory. Furthermore, the circuit consumes energy only when a high voltage is necessary. This limits the total consumption of the integrated circuit.
Preferably, the control circuit comprises a hysteresis circuit to compare the regulation signal with first and second thresholds and provide the result of the comparison at an output of the comparator, and a logic gate comprising a first input receiving the result of the comparison and a second input receiving the control signal, the logic gate providing the closing signal. Preferably again, the closing signal takes a first logic value, which leads to the closing of the controlled switch, when the control signal is in a first logic state and when the high voltage increases and is below a high threshold, and wherein the closing signal takes a second logic value, which leads to the opening of the switch, when the control signal is in the first logic state and when the high voltage diminishes and is higher than a low threshold, or else when the control signal is in a second logic state. Preferably again, the high voltage circuit comprises at least one filtering capacitor to filter the intermediate voltage and a high voltage detection circuit to provide the regulation signal.
To separate the voltage production circuit even more completely from the low voltage regulation circuit that provides the low supply voltage and, at the same time, to increase the quantity of energy received by the smart card, another embodiment of the invention includes the addition of second rectifier and detection circuits to separately supply power to the voltage production circuit. Lastly, the invention is also directed to a smart card comprising an integrated circuit such as the one described above.